Hi Jon, > : (car ((list 1 2 3) 4)) > !? (car ((list 1 2 3) 4)) > 3 -- Variable expected > > Why is a variable expected in this position?
((list 1 2 3) 4) is a highly illegal expression :-/ When PicoLisp tries to evaluate it, it finds that the CAR is a list (list 1 2 3) This CAR is evaluated, to obtain a function to be applied to '4'. This evaluation gives (1 2 3) which is _not_ a legal function, but the interpreter does not check it that far. BTW, (list 'A 2 3) _would_ be a legal function. So (1 2 3) is executed as a function, "binding" the argument '4' to the "formal parameter" '1' (this is the illegal part). For some lucky reason, it did not crash, so that "function" returns '3' (the last expression in the "body" (2 3) of that function). Now 'car' in the original expression gets '3', which is rejected because 'car' can only be applied to a variable (i.e. a symbol or a cell). : (car 3) !? (car 3) 3 -- Variable expected Calling ((list 1 2 3) 4) does not crash immediately, but will surely crash later if used in a real program, because "binding" '4' to the "parameter" 1 destroys the bignum structure of '1'. In the 64-bit version, it crashes immediately because '1' is a short number which is then used as a pointer. The actual fault of PicoLisp is that it doesn't fully investigate whether what it tries to apply to '4' is really a function or not. It doesn't do so for performance reasons, because a complete detection whether a given data structure might be a function or not can be arbitrarily complex. The built-in function 'fun?' does this to a certain level. It says whether a given item "looks" like a function: : (fun? '(1 2 3)) -> NIL : (fun? '(A B)) -> A But it would be too expensive to use it in every possible situation. Cheers, - Alex -- UNSUBSCRIBE: mailto:picol...@software-lab.de?subject=unsubscribe
